Meter for measuring moisture content



2 Sheets-Shee't l P. H. ODESSEY UBTER FOR IIEASURING MOISTURE CONTENT Filed March 5, 1942 WITNESS):

` June 24, 1947. P. H. obEssEY METER FOR 'MEASURING MOISTURE CONTENT Filed March 5, 1942 2 Sheets-Sheet 2 al N AGEN- Patented June 24, 1947 METER FOR MEASURING MOISTURE CONTENT,`

Paul H. Odesscy, Brooklyn, N. Y., assignor, by

mesne assignments, to Portable Products Corporation, Pittsburgh, Pa),

. Pennsylvania a corporation of AApplication March 5, 1942, Serial No. 433,438

(Ci. 17E-183) 13 Claims.

1 Y This invention relates to methods of an' apparatus for measuring variable electrical conditions such as resistance or reactance including-where an appreciable amount of resistance is included. it particularly relates to, but is not limited to.

the measurement of the moisture content of a substance placed between the' plates of a capacitor and .is especially suited to the measurement of hygroscopic substances having relatively' high dielectric losses.

A particular object of the present invention is to provide a measuring instrument which is more stable, more sensitive and' moreV reliable than other known instruments for the purposes for which this invention is suitable. Its apparatus is also simpler and less'expensive than would otherwise be the case as a result ofV the use of multiple-'electronic tubes in such an eiiicient combination, only two tubes being required for its measuring circuit. Another object is to'provide interchangeability of sample-containing test cells and/or capacitors for such cells.

extremely high sensitivity and also stability in- Iii This instrument is an improvement over other t known devices of the dielectric type which also make use of heterodyne or beat frequency methods for detecting a small change of capacity since, ln this instrument, a number of features cooperate to makeit possible'for extremely high sensitivity to be reliably attained without the loss of stability of oscillation, the circuit being so dependable in' its operation that in the large number of cases in which the hygroscopic substance capacity is brought to precisely a standard value hy deformably altering the width' of such cell. Another improved result of the use of mv -improved circuit is that it makes possible for the first time'the use of other improved test cells which may be mounted at al more convenient location somewhat away from. the instrument itse1f.

A n umber of improvements in the measuring circuit of my device which cooperate to provide clude, importantly, the provision of an output limiting means which keeps the amplitude of the difference frequency or beats substantially constant regardless of the changes in the amplitude of the current in the portion of the circuit which includes the test cells capacitor.

Another important improvement is the provision of means for tuning a transformer `for the. difference frequency to provide a high power output of the transformer at a denite beat frequency which is extremely low relative to the standard frequency, thus providing an extremely sensitive and yet stable tuning of the portion of the circuit including the beat meter.

A related improvement is the provision of inductive coupling inthe regenerative portion of the standard frequency circuit, a feature which eliminates a control wbichwould be additionally necessary in case the usual eapacitative coupling were to be attempted in this extremely sensitive device. Still another improvement which materially contributes to the simplicity of the device without interfering with the attainment of high sensitivity coupled with unusually high stability and reliability of operation lies in the selection of multiple-function electronic tubes in the particular structural relations utilized in the improved circuit hereinafter disclosed.

A reliable relation exists between the moisture content of a hygroscopic substance and its dielectric constant in a usefully large number of classes yor substances which include, e. g., granular andv powdered materials as well as liquids.A The measurement in such cases may be accomplishedv by' inserting the substance between the plates of the capacitor so that the dielectric of the `capacitor includes a substance under test with the result that the measuredcapacity of the capacitor will provide a measurement of the dielectric constant and vhence of the moisture content of the substance under test.

In general, this method is practicable except in the rare cases in which objectionable mechanical or chemical characteristics of the substance interfere. 'This technique, especially where-removable test cells are used, has been found'to be simplev and accurate while requiring but little time with oils, shellac, various meals, malts and grains. vA precision of 116% moisture is generally obtainable. Even in cases where the characteristics ofthe substance are so harsh as to cause serious deterioration'of the testvcell, a measurement may be obtained and the test cell discarded after a single test since, following my invention,

standard interchangeable test cells may be used and expended as required. This device is also suited for making purely electrical A measurements.

Changes of resistance have been often used to provide reliable measurements of the moisture content of a wide variety of substances. However, the measurement oi' the dielectric constant` of hygroscopic substances is desirable as being sometimes more reliable than resistance measurements and especially where a, question as to the moisture distribution is involved. In many cases, it is desirable to substantially eliminate both the effect of changes of resistance and the -dielectric eifect of the test cell itself by the use of bare metal side wallsl as the plates of the test capacitor, An object of my invention is to provide a measuring circuit in which the oscillations powerfully tend to be stably maintained even though the substance in a bare plate test cell has relatively high value losses. v

A test cell which is formed exclusively of insulating material makes use of both the capacity and the resistance of the sample in determining the moisture content o f the latter where both the resistivity and capacity eiects correspond with the moisture content. In general, the capacity increases and the resistance decreases as the moisture content increases although no allinclusive law may be stated.

The insulated cell serves as a convenient means for holding certain materials between the plates of a capacitor to there act as dielectric. It also permits the measurement of substances having a much lower resistance than does the bare metal plate cell since, with the latter, the circuit tends to stop oscillating at a somewhat higher, but still low, value of the resistance of the sample than that for the insulated cell. Consequently the insulated cell must be used with samples' of extremely low resistance. As long as the resistance of the sample does not vary with time, accurate measurements arel equally feasible with either the insulated cell or the bare metal plate cell except for the limiting case mentioned above. In either case, the frequency tends to vary inversely as the square root of the capacity.

With the bare plate cell, a variation of the samples resistance at the standard frequency has no appreciable effect upon the equivalent value of the capacity which determines the frequency. In radio frequency oscillators, the resistance of the tuned grid circuit generally has only 'a negligibly slight effect upon the frequency, an effect which decreases as the coupling decreases. Of course with any given coupling, there is a limit to the amount of feed-back power which is available to maintain oscillations and, consequently, there is a limit to the minimum value of the samples resistance at which the circuit will oscillate. In other words, with the bare plate cell, the effect of changes of the samples resistance on the equivalent capacity is negligible but the oscillations will fail or become unstable if this when the samples resistance R1 equals infinity since the capacities are then merely in series in i. e. with the capacity of a sample short-circuited out,

varies with the samples resistance according to the following relation:

e C L... C2 #CIR# where f is the frequencyand the angular velocity w=21rf. Since in the case of certain substances, the samples resistance varies considerably with time and the changes of said resistance are much larger than the changes in the samples capacity, the pure capacity measurement manifestly there provides the more reliable and accurate moisture indication. In` other words, in cases where the resistance of the sample varies with time, the use of the bare plate cell is indicated along with a highly stable oscillator. However it is only fair to note that, with the constant frequency technique disclosed herein, the total effective capacity of the resonant circuit is constant and that consequently the sensitivity of the moisture measurement is substantially the same for both types of cell. i

A further defense of the insulated cell is that it is superior to the bare plate cell in cases where, for example, the resistance change of the sample is greater than the capacity change with variations of moisture content. In such cases, the capacity method disclosed `in the last equation is used in determining the resistance of the sample. An object of the invention is accordingly the determination' of resistance changes by measuring related capacity effects.

Others in the moisture measurement art have employed the bare plate cell by making use of the impedance due to, both resistance and capacity.` However, it would appear at rst that the resistance of the sample would have a relatively much greater effect with the bare plate cell than with the insulated cell since in the bare plate cell the resistance of the sample constitutes substantially the entire resistance while inthe insulated cell, 'the resistanceof the sample is only a small part of the total resistance of the sample plus cell in view of the generally much greater resistance of the insulated cell.

Instead, I have discovered and shown with the aid of the foregoing analysis that. by using a the sample and consequently the total capacity is Y simply the sum of the capacities of the sample and of the insulation. Consequently, the effect ofthe insulation may be fully compensated for,

' when standardizing the instrument, by adjusting a variable capacitor in parallel with both the test cell and the calibrated capacitor. In case any parasitical appreciable capacity effects occur eiIect, while for the other limiting value of Rr=0, where the sample contactsthe insulation, a bare amaca n islmy intention to claim an that I have disin connection with a constant frequency circuit for moisture measurement by a purely capacita.- tive method is thus further apparent. f

Accordingly, an object of my invention is to develop a moisture meter of the bare plate type by including an unusually stable oscillator'circuit so that the greater inherent accuracy of the bare plate cell may be had over much-wetter samples than heretofore or for cases' in-which the samples resistance varies considerably with time.

The measurement of the capacity is made by means of the substitution method. In making the test, the instrument is iirst standardized using an empty test capacitor, thenf a'speciiic weight of the substance is placed between theplates of the test capacitor and the instrument is restored to the initial standardized condition, with the sample between the capacitors plates, by means of a calibrated variable capacitor. The reading of this calibratedcapacitor is converted into percent moisture content by means of tables compiled for the material under test.

The measurement is made at an intermediate radio frequency and makes use of the heterodyne or beat frequency principle of detecting small changes of capacity. Exclusive of the regulated D. C. power supply. the circuit consists of two radio frequency oscillators respectively of variable frequency and of constant frequency. and a heterodyne detector capable of detecting the difference frequency existing beween the two oscillators, and including means for observing these differences visually on a meter.

'Any reasonably high potential source of direct current may be used to operate an electrical measuring instrument of this class. It may include batteries within the instrument casing or the D. C. supplymaybe derived from an A. C. supply by means of a rectifier power unit. Such a power unit consists of a transformer which supplies power to a full Wave rectier Whose output is filtered by means of an inductance and a capacity. The poweroutput of this unit may be regulated by means of a resistor shunted by a gas discharge tube between the output terminal of the' resistor and the ground, such terminal serving as the positive point of the D. C. power supply to the capacity measuring instrument. such a power supply unit. A. C. power is converted'into D. C. power having only a small A. C. ripple component. The combination of such a resistor and gas discharge tube reduces variations in the output voltage so that they are small compared with those of the voltage ofthe A. C.

supply. Thispower supply unit is shown only diagrammatically since it is known to those skilled in the art.

Thus the chief objects are to provide methodsA and means for causing such a highly sensitive capacity meter, particularly a moisture meter of a dielectric type, Vto stably and reliably operate and particularly with respect to minimizing the damping eect due to a lowering of the resistance in the portion of the measuring circuit including the test capacity or, in other words, due to increased dielectric losses.

`'I'hese and such other objects of the invention will appear to those'skilled in the art from the accompanying drawingsA and specilication in which is illustrated anddescribed by way of ex; ample a specic embodiment of the invention.

closed that is of a. patentable nature. Fig. 1 is a. diagram of the preferred circuit in which the preferred insulated form of a test cell for hygroscopic material .is ijagrammatically shown.

Fig. 2 is a view, generally in perspective, of the outside of the moisture meter. It has a corner broken awaykto show the capacitor for the test ceu.'

Fig. 3 is a View, generally in perspective, of the test cell preferably used in the moisture meter shown in- Fig. 2 with samples'whose loss corresponds with the mositure content, one corner of its cover being shown broken away in Fig. 3. It

'is also preferably used with samples having such high loss as to damp out the circuits oscillations with a bare metal plate cell.

Fig. 4 is a cross section of the test cell, shown in Fig. 3, and its cover taken at the mid section.

Figs. 5-7 are diagrammatic views of different forms of improved test cells in which bare metal plates are used in the capacitor and the hygroscopic material comes into direct contact with such plates, the preferred type for samples having losses varying with time.

Fig. 5 shows a two-plate capacitor with the outer plate grounded and the inner plate insulated from ground,

Fig. 6 shows a three-plate capacitor with the outside plate grounded and the shorter central plate insulated from ground.

Fig. 7 shows a generally cylindrical form in y which the outside cylinder is grounded and the detector 4 which includes a meter 5 that is sensi- By the use of tive t0 the heterodyne output, of detector 4.

Variable frequency oscillator This isy in part a generally conventional radio frequency feed-back type oscillator. It comprises an yoscillator coil 6, one terminal aof which is connected by the line 'I with one side of: the test capacitor Cx, a calibrated variable capacitor C1 and a frequency-standardizing capacitor C2, the other sides of these capacitors being connected to the ground. Line T also connects terminal a, of oscillator coil 6 through the grid leak resistor 8 and its by-pass capacitor 9 to the control grid vIII of the triode section of tube V1. The heating lament of this tube is connected to any suitable source of power, e. g. a. battery as shown, and its cathode II is connected to ground. The other terminal b of oscillator coil 6 is connected to one side, at terminal c, of a tuned output circuit which is loosely coupled to the oscillator ccil A coil 2li for the triode section of tube V1.

` capacitor I1 connected across coil 'I2 which is loosely lcoupled with' oscillator coll i. One side of coll Iland'of. capacitor I1 has a common terminal c while the-'other side of |2 and I1 has a commonzterminal d connected by line l 3 with the hexode control grid I9.

Also coupled to oscillator coil 5 is the feed-back Cne terminal e of coil 2|I is connected to the plate 2| of the triode section of tube V1 and the other terminal f of coll 20 is connected by line 22 to the ground through a by-pass capacitor 23. Line 22 also serves to connect the positive terminal y of the power supply unit I with the plate 2| of the triode section of tube V1 through coil 20. The capacitor 23 provides a low-impedance path for radio frequency components to the ground while allowing the D. C. supply to be carried to the circuit which includes plate 2| without being short-circuited to the ground. It is seen that the circuit of the variable oscillator is completed through the grounding of capacitors C11, C1` and C2 and through the cathode of tube V1.

The frequency of the variable oscillator is varied by changing the capacity of any or all of the capacitor Cx, C1 and Cz. In its use in capacity measurement, the frequency is adjusted to a standard or xed value by means of the capacitor Cz while the calibrated capacitor C1 is at its zero reading with an empty test cell 24 between the plates of capacitor Cx. A known weight of the substance under test is put into the test cell 24 which is then placed between the plates of capacitor Cx. This alters the frequency of the variable frequency oscillator 2 from that for the empty test cell. The operator then manually adjusts the calibrated capacitor C1 to restore the frequency to its initial standard value.

Constant frequency oscillator The crystal 25 controls the standard frequency oscillator 3 and has one side connected to ground and the other terminal h connected through resistor 26 to ground. Terminal h is also the terminal of primary coil 21 of the feed-back transformer. The other end i of coil 21 is connected by line 29 with the control grid 29 of the triode section of tube'Vz. 'I'he secondary coil 30 of the feed-back transformer, whose primary is coil 21, is tuned to the crystal frequency by means of a capacitor 3|, one side of which is connected by of the crystal controlled oscillator 3. The combination 4| and 42 is necessary for the operation of the hexode portion in order to establish proper operating conditions for the hexode section of tube V2. The triode section of tube Va and the constant frequency oscillator 3 as a lwhole are designed. to be stable over a wide range between extreme operating conditions and to operate at the natural frequency of the crystal over such range.

Heterdyne detector Plate 43 of tube V2 responds to the component frequencies developed through the electronic interaction of grid 44, that is directly coupled to the standard frequency oscillator 3, with grid I9 which is loosely coupled to the variable oscillator 2. In other words, portions of each of the outputs of the variable frequency and the xed frequency oscillators are mixed by combining electrons in the space between the cathode 31 and the plate 43 of the hexode converter section of tube V2.

This is accomplished by connecting the line I8, as earlier mentioned herein, to the loosely couple tuned output circuit of the variable oscillator, at the terminal d of coil I2 to be specific. v

Part of the constant frequency output of the crystal oscillator is brought into the aforementioned inter-electrode space by means of a supplementary grid 44 connected to the control grid 29 of the triode section and located between the cathode 31 and the screen grid 38 of hexode section of tube V2. The reaction of the output of both oscillators results in many frequency components, one of which is the difference frequency; the other components, having a much higher frequency, by-pass -to the ground by means of a capacitor 45 to which plate 43 is connected by line 46.

The output of plate 43 is also carried by line 46 to one end 7' of the primary coil 41 of audio transformer 48, the other end lc of coil 41 being connected by line 22 to the positive terminal g of the direct current supply unit Primary coil 41 is tuned to a low audio frequency, e. g., approximately 100 cycles per second, by means of the v fixed capacitor 45. This is but one way of obtainline 32 with plate 33 ofthe triode section of tube with respect to its cathode 31 which is adjacent a. heating filament which is supplied from any convenient source of current,1 e. g. a battery as shown. Line 34 also is connected both to the screen grid 38 and tothe suppressor grid 39 of the hexode section `of tube V2.

The cathode 31 of tube V2 `is common for both the hexode and triode sections and said cathode is connected to ground by line 40 through a self-- biasing combination consisting of resistor 4| and its bypass capacitor 42, such connection to ground,

.of course, being without eilect upon the frequency ing a tuned resonant circuit; another would be the tigning of the circuit including the secondary coil One side of the secondary coil 49 of the audio transformer 4B is connected by line 50 with the plate 5| of the diode rectier sectiony of tube V1. The other end of coil 49 is connected by line 52 through a resistor 53 in series with the D. C. milliammeter 5 t o ground, line 52 being also connected to ground through a filter capacitor 54, the circuit being completed through the cathode of tube V1. Capacitor 54 acts as a low impedance path to ground for the A. C. components of the rectilied audio frequency output While the D. C. component must flow exclusively through resistor 53 and meter 5.

The circuit of the heterodyne detector 4 thus includes the secondary 49 and the D. C. milliammeter 5. The function of this portion of the heterodyne circuit is to rectify the A. C. output of the audio transformer 48 so that the D. C. milliammeter 5, Whose maximum capacity is preferably of the order of 1 ma., provides a visual measurement of the difference frequencies which originate in the hexode converter section of the tube V2.

While the visual `indication of meter 5 has been 9 y shown to 'enablethe operator to manually adjust capacitor C1 of the variablefrequency oscillator, it will be apparent to those skilled in the art that this may be done automatically instead of manually and also that, instead of adjusting Cr, the moisture input to a continuous process might be directly governed by the milliammeter 5, e. g.,

through photoelectric or other means which would not interfere with the operation with the D. C.

milliammeter 5.

When the'diiference frequency is in the neighborhood of zero and is equal to 100 cycles per second, the resonant frequency of the primarywinding 41 of audio transformer 48, the output of transformer 48 will be a maximum and consequently the reading will also then be a maximum.

Since the difference frequency is the same for frequencies of the variable oscillator either I slightly greater or less than the frequency of the oscillator 3, the meter will first indicate a maximum deflection, then zero followed by a maximum again as the frequency of the oscillator 2 is changed continuously-in either direction over this range. Zero deflection of meter 5, falling between the two maximum deecti'ons obtained in theV above manner, uniquely indicates that both oscilioV 4 to the grid Il of tube V1 and for the radio free quency component of the current of said grid.

Upon placing a sample ofhigh dielectric loss between theV plates of capacitor Cx. there is a corresponding reductionV of uthe amplitude ofthe 1 generated radio frequency. voltage. oscillations across Cx and of the radio frequency component of the voltage across primaryv coil 6 and hence a proportional reduction of the radio frequency voltage across its secondary coil I2, thereby def creasing the radio frequency-voltage'applied to the control gridIG of tube Vn, which effect taken alone wouldtend to reduce the amplitude of the voscillations of the output from hexode plate 43 of tube vl which is controlled by the grid I6. But the inclusion of the resistance I4 and its by-pass capacitor l5, for the grid current of tube ,Vi, in the circuit of the control grid I6 ofthey tube -Vz has a compensating eiect which is brought about as follows:

The reduction ofthe amplitude of the generatedradio frequency voltage across Cx is also accompanied by'a proportional reduction in the lators 2 and 3 are operatingat the same frequency and that the difference frequency is zero.

Due to the tuning of the resonant circuit consisting of condenser 45 and primary winding 41, maximum deections occur in rapid succession about 200 cycles apart (approximately 0.04% of d. c'. current to grid l0 of tube Vr.- AConsequently the voltage dropacross resistor I4 likewise decreases and hence correspondingly reduces the negative bias of the control grid I6 of tube V2.

shifts the cut-olfvoltage to increase the 'Y gain of the output circuit. of the converter tube.

thstandard frequency of 465,000 cycles per'seco which has been used by way of example) as e difference frequency is varied continuously in meter deflections very close to the standard freeither direction. The occurrence of maximum quency greatly simplies and facilitates the 'ad-1 justment of the frequency of the variable oscillator to the standard frequency since in this manner the approaches to, and-deviations from, the standard frequency are thereby visually amplified. This provides far sharper tuning than the conventional method of amplifying the output without changing the spacing of the peaks 'of the out- 'put and the width of the intervening valley.

The general heterodyne peaks-and-valley c'ri' teria of detection is used occasionally in the prior art to determine that the frequencies of the two oscillators are precisely equal. By the hereindisclosed method, extremely small diierenc'es of frequency can be readily detected with great sensitivity. Equality of the two frequencies serves as a basic reference point in the measurement of capacity.

The measuring circuit isinitially adjusted to this condition with an empty test cell in capacitor Cx and, then after the cell containing the sample is inserted between plates of capacitor Cx, thereby altering the frequency of the variable oscillator` 2, the circuit is restored tothis same condition by means of calibrated condenser C1. The difference between the two settings of C1 is a measure of the capacity change introduced bythe sample under -test and hence a measure of its moisture content.Y

Output control means -Now that the measuring circuits as a'whole have been described, the output control feature, which is provided by the inclusion in the circuits of grids I0 and I6 of the resistor I4 with its by-pass capacitor I5, may -be further described as follows: A

As noted earlier. the resistor I4 and capacitor I5 respectively form paths for the d. c. current The same function may be otherwise attained as by making use of. e. E, anon-linear charac` terlstlc of the converter-tube. As a result, the output from plate 4 3 of ltubeVzwould increase if it were not for the fact that the amplitude of 'the generated radio'frequency voltage across C.

and the amplitude of the radio frequency signal to be impressed upon the control grid I6 were also reduced as stated in the preceding paragraph.

- The net and important result is'that one effect substantially off-sets Vthe other and eliminates,

grid I6 of tube V2. if present, is relatively much smaller and helps to' keep constant the output from tube V2 with a negligible effect vupon the One skilled in the art,

functioning of tube V1. when facing any particular application, can readily t suitable values ,of the resistances and Acapacities of resistors. 8 and I4 andtheir by-pass capacitors 9 and I5 respectively. For example in a typical moisture meter, the Avalues are as follows: resistor 8-250,Il00 ohms, resistor I4 20,000 ohms, capacitor 9-70 mmf.,'and capacitor I5-0.1 mf.

By using the automatic bias control` described,

-no adjustments are necessary to increase ordecrease the sensitivity of the meter 5v which otherwise would deflect' weakly or go off-scale in the moisture measurement of hygroscopic substances whosedielectric losses are respectively high or low. 1

Operation ofthe device of Fig. 1.

As earlier mentioned herein, the v arlableoscil lator is iirst standardized with the test capacitor Cx empty and the calibratedcapacitor Cr `set at l itszero which corresponds with its maximum capacity, Capacitor Cris then adjusted to make l1 the frequency of the variable oscillator equal to that of the fixed oscillator. This equality is determined by continuing the adjustment until there is obtained a null reading of the meter 5. The

null reading falls between two successive maximum readings as the capacitor is turned in one direction. After the circuit has been standardized. 'capacitor C2' is 'left undisturbed throughout the remaining procedure.

A specific weight of the hygroscopic sample is then placed between the plates of C; and the null reading of meter is again obtained by similarly adjusting the calibrated capacitor. The

" reading of the scale for this calibrated capacitor indicates the difference between its initial and capacity' change introduced by the test sample. By testing samples of the same substance while varying its moisture content, a conversion table is obtained giving the relation between moisture' content and capacity of the substance under test.

This empirical table is .then used in testing for' the moisture content of other samples of the same substance.

It is also noteworthy that the tuning of the this arrangement so that, in the neighborhood of the standard frequency, the meter'acts very sensitively Vwith respect to small frequency changes. Due to this fixed tuning, the change from null to maximum meter indication is brought to within approximately 0.02% in the instrument discussed by way of example since in suchcase the resonant audio frequency is only aboutA 100 cycles per second while the standard constant radio frequency is 465,000- cycles per` second.

From the foregoing description of the device and its operation, it will be evident to those skilled in the art that such a `stable, highly sensitive oscillating circuit has been provided, particularly by the output control feature, as to make possible the use of reliable moisture measurements of a hygroscopic substance between the bare plates of a test capacitor by the use of purely capacitative relations in spite of heavy damping due to a high dielectric loss (i. e. a low shunting resistance) of the material between the plates of the test capacitor Cx. However, the volume limiting combination I4 and I5, the stable and eilicient inductive coupling of the oscillator coils 21 and 30 for the standard frequency oscillator 3, and the use of a tuned audio Atransformer'48-of the heterodyne detector 4 all cooperate to provide an lmproved circuit which tends to stably oscillate and hence to provide a beat meter 5 to enable highly accurate readings of the calibrated condencer C1 to' be made in spite of `either such a high loss in or such a low shunting resistance for test capacity 'is final settings and, as such, is a measure of the 12 which for convenience is provided with a detachable cover 6l. A metal p1ate62 acts as a top side of case and is provided with a metal door 63 which is grounded by its hinges upon the metal plate 62. Supported by the plate 62 is the milliammeter 5 in a location to be readily visible to the operator while he manipulates with his right hand the knob 64 of the calibrated capacitor Ci or vknob of capacity C2 for standardizing the instrument with an empty test cell between the plates vof the test capacitor Cx whose capacity When empty is set to a standard value by use of the adjustable screw-end of line 1 shown in Fig. 2. A switch 66 is provided for the entire power supply including unit l. Pilot light 61 serves to indicate whenever the power is on in power supply unit I. A plate 68 of operating instructions is also provided. The outer plate 69 of capacitor Cx is connected by line 10 with a terminal 1| on the thin metal ground sheet 12 which preferably adheres to the inner walls of casing 60 and is in substantially conductive contact with the metal top 62 and its door 63. This shielding arrangement completely eliminates radiation or reception of radio frequency impulses as well as the effect of stray capacities of external objects in the vicinity of the instrument.

Casing 60 is provided with a cover 13 for a service entrance and clean-out opening, i. e. cover 13 is removable to permit the interior to be accessible for inspection and for maintenance and at the same time to enable an operator to clean out any material which may have fallen into the space around the test capacitor Cx. The casing is also provided with two screened air vents 14 which permit heated air to escape and hence serve to keep the instrument cool with the result that its stability is likewise increased.

The inner plate 15 of test capacitor Cx is insulated from the outer plate 69 by insulating blocks 16 upon which both plates 15 and 69 are mounted. Plate 15 is connected by line 1 with the grid leak 8 for the variable frequency oscillator triode section of tube V1. Wooden wall 11 is not provided with any metal sheet.

'I'he test cell 24 and its cover 8| are lshown in Figs. 3 and 4 and are made of dielectric material in the preferred embodiment, a fixed weight of sample being inserted in the test` cell when a measurement is desired. Test cell 24 is provided with a cylindrical` spacer block 82 with one or more spacing washers 83, to provide a standard cell-capacity, inserted between the Walls of test cell and the block 82.v These elements are then fastened by means of screws 84, after which the cells are finished and interchangeable, i e., have azimuth, from that for which the cell has been Cx as would damp out the oscillations of any earlier known capacity meters of the heterodyne type. But the invention is not at all limited to capacity-type meters having bare metal plates in contact with the dielectric. Instead, the mai-n commercial field uses the preferred embodiment in which the test material is put into a test cell molded from an insulating plastic. 4

In Figs. 2-4 is shown the preferred embodiment of the invention. For convenience, the entire power supplyand measuring instrument may be combined as indicated within a single case 60 standardized. The cover is provided with a small hole 86 in which a thermometer may be placed so that a temperature correction may be made for the measurement of the substance under test.

The use of this convenient test cell, which is instantly removable from the instrument, permits great iiexibility in the handling of liquid or solid test samples. This cell isv also useful in developing methods for certain materials that exhibit peculiar properties and is of advantage in obtaining uniform packing for granular and powdered materials since the loaded cell can beshaken in in each instance.

13 astandard mannerbefore the test. It also allows great versatility in handling of samples, weighing, and the setting up of a large number of samples in test cells for moisture determinations with economy of time and eiort. Fig. 1 shows diagrammatically this preferred test cell in-the circuit disclosed.

The technique of measuringmoisture by means of this instrument is simple and requires little time. Notable also is the versatility of the instrument, its use being practicable not only for a great variety o! materials but also for purely electrical measurements.

Figs. 5-7

These snow bare plate modincauons or a test cell. In the tt cell oi! Fig. 5, the outer plate 90 it is n advantage be able to shake the test cell bodily while taking the readings. In

bottom of the valley. By providing a tuned low is connected by spring connection 9| (shown by l dot-dash lines) to the metal ground sheet 12. The inner plate 92 is connected by spring connection 03 (likewise-in dot-dash lines) to line 1 which is connected as before to the grid leak 8 of the triode section of tubeV1. Plates 90 and 92 are physically connected to a molded insulated spacer 94 which seals the sides and bottom of the test cell to cause the test cell to form a cup. The upper ends of spacer 9| are provided with lugs 85 which rest upon corresponding shelves in the casing 60. The construction is such that, as with the cell of Figs. 1 4, an operator can 'manipulate the test cell as required in lling it and thereafter can readily place the test cell in position ,for .measurement and later remove it readily.

Another bare plate modification of a rectangular test cell is somewhat diagrammacally shown in .Fig 6. In this modification, outer plates |00 are provided and together detachably electrically connected to an outer conductive covering I| of a coaxial cable |02, covering |0I. being grounded on metal sheet 12. An inner plate |03 is somewhat shorter than plates |00 and is detachably electrically connected to the central conductor |04 of the coaxial cable |02. Molded in-V sulation generally corresponds with that 9| of the cell of Fig. 5.

At the other end of the coaxial cable, the covering 10| is similarly detachably electrically connected to the instruments grounding sheet 12 and the central conductor |04 is similarly connected with line 1. This use of a flexible coaxial cable has the considerable advantage in many cases of permitting samples to be tested in a location which is more convenient for sampling purposes than that of the instrument itself. To prevent radiation eiects, a metallic cover and shielding for the end walls are provided although not shown.

The. modification of Fig. 'I is generally similar to that of Fig. 6 as to the use of the coaxial cable and an inner plate, the outer plate or plates being grounded in both cases and separated fromV the inner plate by a molded insulation separator |06. A metallic cover |09 is provided. In the embodiment of Figf, the outer plate |01 is made cylindrical but is detachably electrically connected with the conductive covering of the coaxial cable generally as in Fig. 6, a conventional screw connection being slightly less diagrammatically shown however. The plate 08 is a 'domed cylinder mounted concentrically cylindrical plate 101 and electrically connected with central conductor |04 of the coaxial cable generally as in Fig. 6. The detail of the detachable platev with the coaxial cable is somewhat diagrammatic For a certainclass of subaudio frequency (of e. g. cycles per second) in the loadV network, El have proceeded in the opposite direction Vfrom the known teachings of the audio receiving art in which efforts are made to reduce the values of inductance and capacity to a minimum so that the resonant audio frequency will be raised well above 5,000 cycles per second with the result that harmonics will not be unduly distorted. In such receivers, the low audible note of 10|) cycles per second in resonance would of course be intolerable.

The terms and expressions which I have employed are used in the specication as terms of description and not of limitation, and I have no intention, in the use of terms and expressions in the claims, of excluding any equivalents of the features shown and described and portions there of, but recognize that various modications are Y possible within the scope of the invention claimed.

It is clear from the foregoing method of operation that the means for adjusting the frequency A of the variable oscillator to the standard value 'can be otherwise attained as, e. g. -by usingV the graduated variable capacitor C1. In this alternative case, the index of the graduated variable ca'- pacitor may be shifted to provide a zero reference index for the measurement. The terms insulation-cell as used in the claims and insulated cell as used loosely in the foregoing speciiication can be regarded in a general sense more specically as a cell substantially entirely-consisting Y 1. In a deviceof the measuring and controlling class which is sensitive to the capacity of an element, the combination of a standard frequency oscillator; a variable frequency oscillator having a. first electronic tube and including a circuit for regeneratively applying an oscillating'voltage'to said element at a frequency -which depends upon the latters capacity, and means for bringing the Vvariable frequency'l to the standard wthupon an increase of the energy loss in said element, a decrease both of the amplitude of the stated voltage oscillation, of the average value of the D. C. current to the control grid of said tube, and of the oscillating output of the variable frequency oscillator; a heterodyne circuit having an electronic vfrequency converter tube, an output circuit, and a circuit 'coupling the converter tubes control grid with the output of said rst tube so that the converter tubes grid voltage oscillations tend to decrease in amplitude proportionally with a decrease of the oscillating output of said variable frequency oscillator whlch causes the amplitude of the oscillating output of said converter tube to decrease; and a control device for maintaining substantially constant the oscillating output of said converter tube regardless of changes of the stated energy loss, which control device comprises a resistor and a frequency component by-pass capacitor, said control device being connected in the control grid circuits of both said tubes and tending, upon a decrease of the average value of the D. C. grid current of said rst tube and hence a decrease of the stabilized average value of the negative D. C. voltage in said circuit to said converter tubes control grid, to proportionally decrease the negative D. C. bias of the last mentioned grid and hence to proportionally increase the gain of the output circuit of said converter tube, whereby, upon an increase of said energy loss in the element, the earlier stated tendency to decrease the oscillating output of said converter tube is compensatingly offset by the later stated simultaneous tendency to increase the oscillating output of said converter tube.

2. In a device of the measuring and controlling class which is sensitive to the capacity of an element, the combination of a standard frequency oscillator; a variable frequency oscillator having a first electronic tube and a circuit for applying.

an oscillating voltage to said element at said standard frequency; a heterodyne circuit having an electronic frequency converter tube and a circuit coupling its control grid with the output of said first tube; and a control device electrically connected to said control grid and governed by the D. C. grid current of the rst tube to automatically maintain substantially constant the oscillating output of said converter tube regardless of changes of the energy loss in said element.

3. In a device of the measuring and controlling class which is sensitive to the capacity of an element, the combination of a standard frequency oscillator; a variable frequency, oscillator having a first electronic tube and a circuit for applying an oscillating voltage to said element at said standard frequency;` a heterodyne circuit having an electronic frequency converter tube and a circuit coupling its control grid with the output of said first tube; and a control device for automatically maintaining substantially constant the oscillating output of said converter tube regardless of changes of the energy loss in saidA element, which, control device comprises a resistor and its frequency component by-pass capacitor,

said control device being connected in the control grid circuits of both said tubes with said resistor effective as at least a portion of the grid leak of said first tube.

4. In a device of the measuring and controlling class which is sensitive to the capacity of an element having appreciable radio frequency energy loss, the combination of a standard radio frequency oscillator; a variable radio frequency oscillator having a rst electronic tube and a circuit for regeneratively applying an oscillating voltage to said element at a frequency which depends upon the latters capacity, and including means for bringing the variable frequency to that of the standard frequency oscillator; a heterodyne circuit having an electronic frequency converter tube and a circuit coupling its control grid with the output of said rst tube; a control device governed by the grid current of the rst turbe to automatically maintain substantially constant the maximum oscillating output of said converter tube regardless of changes of the energy loss in said element; a meter whose indication depends upon the value of an impressed voltage; and means coupling said radio frequency oscillators with said meter and including a network having a transformer whose primary coil is connected with an output portion of said heterodyne circuit to vary the voltage across said primary coil in accordance with` the difference between the variable and standard radio frequencies, a secondary coil connected to said meter to impress a voltage across said meter, and means for tuning the transformer to operate efficiently at a low audio frequency to provide a minimum voltage across the meter when the variable and standard radio frequencies are identical and a maximum voltage when the difference frequency is substantially equal to the stated tuned audio frequency.

5. A device of the measuring and controlling class which is sensitive to resistivity and dielectric effects of a sample whose resistivity and dielectric effects correspond with the moisture content, the combination of a standard radio frequency oscillator; a capacitor including a cell for containing a predetermined amount of the sample said cell being constructed of insulating material to have a substantial portion of the insulating material as a dielectric in series with the sample in the field of the capacitor; a variable radio frequency oscillator having a rst electronic tube and a circuit forregeneratively applying an oscillating voltage to said capacitor at a frequency which depends on the latters capacity and an empty or loaded cell therein at said standard frequency; a heterodyne circuit having an electronic frequency converter tube and a circuit coupling its control grid with the output of said rst tube; a control device governed by the grid current of the first tube to automatically maintain substantially constant the oscillating output of said converter tube regardless of changes of the energy loss in said element; and a meter connected to an output portion of the heterodyne circuit to indicate the oscillating output of the converter tube and hence the difference frequency; a portion of the variable frequency oscillator being adjustable in accordance with the stated indication of said meter to reduce the difference frequency substantially to zero, whereby the stated adjustment of said variable frequency oscillator portion corresponds with the moisture content.

6. In a device of the measuring and controlling class which is sensitive to dielectric effects of a sample whose dielectric effects correspond with its moisture content, the combination of a standard radio frequency oscillator; a detachable cell for containing a predetermined amount of the sample; a variable radio frequency oscillator having a first electronic tube and a circuit for applying an oscillating voltage across said cell when the cell is attached to the device as a reactive portion of said circuit whose capacity affects the variable frequency; a heterodyne circuit including the output of the standard frequency oscillator and having an electronic frequency converter tube and a circuit coupling the converter tubes control grid with the output of said first tube; a control device governed by the grid current of the first tube to automatically maintain substantially constant the maximum oscillating output of said converter tube regardless of changes of the energy loss in said sample; and a meter connected to an output portion of the heterodyne circuit to indicate the oscillating ,17 output of the converter tube and ference frequency: a portion of the variable frequency oscillator being adjustable in accordance the cif-` ance with the stated indication of said meter` to reduce the difference frequency substantially to zero, whereby the stated adjustment of said variable frequency oscillator portion substantially corresponds -withthe moisture content as it affects the dielectric effects of the sample in saidcell.

7. In a device of the measuring and controlling class which is sensitive to resistivity and dielectric eiects of a sample whose resistivity and dielectric eilects correspond` withits moisture content, the combination of a standard radio frequency oscillator;l a detachable bare metal plate capacitor cell for vcontaining a predetermined amount of the sample and constructed to have a negligible portion of insulating material as a dielectric in series with the sample in the vileld of the capacitor; a variable radio frequency oscillator having a rst elecronic tubeand a circuit for applying an oscillating voltage across said cell when the cell is attached to the device as a reactive portion of said circuit whosejcapacvity affects the variable frequency; a heterodyne circuit including the output of. the standard frequencyy oscillator and having an electronic frequency converter -tube and a circuit .coupling the converter tube's control I grid with the' output of said iirstvtube; a control device governed by the grid current of the first tube to automatically maintain substantially constant the maximum oscillating output of said converter tube regardless of changes of the energy loss in said element; and a meter connected to an output portion of the heterodyne circuit izo-indicate the oscillating output of the converter tube andhencethe difference frequency; a portion of the variable frequency oscillator being adjustable in accordance with the stated indication of said meter to reduce the difference frequency substantially to zero, whereby the stated adjustment of said variable frequency oscillator portion substantially corresponds with the moisture content as it affects the dielectric eie'cts ofthe sample in said cell and is substantially independent 'of variations of the resistivity of the sample.

8. In a moisture meter for hygroscopic substances and including a radio frequency device whose frequency is vsensitive to changes in the capacity of a capacitor, 'the combination of a cell constructed of vinsulating material to contain the sample to tested and to be readily insertable between the plates of said capacitor, shielding completely surrounding said device and said capacitor and electrically connected to one portionof said device and including a door of shielding connected to the remainder. of the shielding and. removably closing an opening in such shielding through which the cell may be inserted between the plates of said capacitor, one plate of the capacitor being adjacent to the shielding and connected electrically therewith and the otherplate of the capacitor being adjacent the device and connected electrically with another portion of the radio frequency device so that the capacitance of said capacitor affects the frequency of said device, andmeans for varying the spacing of the plates of the capacitor to very precisely predetermine the electrical Vcapacity of the entire capacitor. i I

9. The combination set forth in claim 8 lncluding means for deforming one of said plates 18 to adjust the spacingbetween said plates to a predetermined value.

10. A- device of the measuring and controlling class which is sensitive to an impedance effect of a sample comprising, in combination, an electrical measuring instrument, a cell constructed to contain a predetermined amount of the sample and to be readily attachable to and detachable from said instrument. and a shielding means completely surrounding said instrument and said cell when the cell is in a testing position; said instrument comprising: a standard radio frequency oscillator; a variable radio frequency oscillator having a first electronic tube and a circuit including a two-element means for applying an oscillating voltage across said cell when the cell vis attached to the instrument inthe testing position, said two-element means being constructed to then have the respective ones of said elements electrically connected with the shielding-means and with a portion of the variable frequency oscillator which governs the frequency of the latter to aiect the latters frequency in accordance with the samples stated impedance effect, and means for bringing the variable frequency to the standard frequency; a heterodyne circuit having an electronic frequency converter tube and a.

circuit coupling its control grid with the output a meter whose indication depends upon the value l of an impressed voltage; and a network havingk a transformer whose primary voltage varies in accordance with the output of the heterodyne circuit and hence with the diierence between the varia-ble and standard radio frequencies, a secondary coil connected to said meter to impress a voltage across said meter, and means for tuning the transformer to operate efficiently at a low audio frequency to provide a. minimum voltage across the meter when the variable and standard radio frequencies are identical and with a maximum voltage across the meter when the difference frequency is substantially equal to the stated tuned audio frequency.

11. The combination set forth in claim 1 in which said resistor has such a value that each swing of the oscillating voltage of the control -of the converter tube is varied beyond the cutolf value of the converter tube. the combination including a resistor in the cathode lcircuit of the converter tube for determining said cut-off value.

12. In a moisture meter for hygroscopic subcapacitor plates to then be subject to the radio frequency field between'said plates to then vary said frequency in accordance with the value of an electrical characteristic of the sample which corresponds with the moisture contentwhen said cell is loaded with a standard weight of sample, and shielding completely surrounding said device and including a door of shielding material connected to the remainder of the shielding and constructed to be movable for admitting the cell to an operating position within the shielding and for removing the cell from the shielding, said capacitor plates being electrically connected with the radio frequency device to apply the said radio frequency voltage to the cell 'when the cell is in the operating position tothen impose said radio frequency eld across the sample when the cell is loaded, -said plates being constructed and arranged to have vone plate of said capacitor adjacent the shielding and also electrically connected therewith and to have the other plate of said capacitor adiacent'said device and connected with a portion thereof which governs the frequency.

13. In a moisture meter for hygroscopic substances and including a radio frequency device zwhose frequency is sensitive to changes in the capacity of a capacitor, the combination of a cell constructed of insulating material to contain the sample to be tested and to be readily insertable between the plates of said capacitor, said cell having walls relatively adjustably deformable to very precisely predetermine the electric capacity of the cell and sample when said cell is loaded with a standard weight of a standard reference substance, and shielding completely surrounding said device and said capacitor and connected electrically to one portion of said device and including a door of shielding material electrically connected to the remainder of the shielding and removably closing an opening in such shielding through which the cell maybe inserted between the plates of said capacitor, one

plate of the capacitor being adjacent to the shielding and also connected electrically there vwith and the other` plate of the capacitor being adjacent the device and connected electrically with another portion of the radio frequency device which governs the frequency.

' PAUIr H. ODESSEY.

REFERENCES CITED 1 VThe following referencesare of record in the ille of this patent:

UNrrED sTATEs PATENTS Number Name Date 2,266,114 Bartlett Dec. 16, 1941 2,282,092 Roberts May 5, 1942 1,983,665 Hickok Dec. 11, 1934 2,254,023 Wright et al Aug. 26, 1941 2,076,441 Berry Apr. 6, 1937 2,069,046 Rabezzana Jan. 26, 1937 1,878,109 Clark Sept, 20, 1932 2,043,241 Eyer June 9, 1936 2,162,335 Jacob June 13, 1939 2,086,892 Barton July 13, 1937 2,276,672 Roberts Mar. 17, 1942 FOREIGN PATENTS Number Country Date Great Britain July a1, 19'33 

